What effect do each of these have on the freezing point of the solvent?
a. a nonvolatile solute that dissociates in the solvent
b. a volatile solute that does not dissociate
c. two solutes that react according to the equation:
A+B---> C
Please explain why.
a) The freezing point will decrease the most (compared to parts "b" and "c". If the molality of the solute is m, the lowering of the freezing point will be
deltaT = 2•Kf•m (the effective molality is 2m)
b) The lowering of the freezing point will be
deltaT = Kf•m (compare this to part "a").
c) If the molality of C is m,
deltaT = Kf•m (compare this to part "a" and "b")
Comment of Part C:
Normally the effective molality when we have two solutes would be the SUM of the two molalities:
m(effective) = m(A) + m(B)
However since A and B combine to form C, if we use equal moles of the two solutes, the effective molality will be equal to the molality of ONE of them which is equal to the final molality of C. If we mix A and B at a mole ratio not equal to 1, things get more complicated since the effective molality equals the molality of C plus the final molality of the excess reagent.
That is not a well thought out question unless it was constructed to stimulated thinking about all possible scenarios.
To understand the effect of different solutes on the freezing point of a solvent, we need to consider the concept of freezing point depression.
Freezing point depression occurs when a solute is added to a solvent, causing the freezing point of the solvent to decrease. This is because the presence of the solute interferes with the formation of stable crystal lattice structures, which are responsible for the solid phase of the solvent.
Let's explore the effect of each scenario on the freezing point:
a. A nonvolatile solute that dissociates in the solvent:
When a nonvolatile solute dissociates in the solvent, it forms multiple particles or ions. The presence of these additional particles increases the effective concentration of solute particles in the solution, and according to the colligative properties, the freezing point of the solvent is lowered proportionally to the concentration of particles. Therefore, a higher concentration of solute particles will cause a greater depression of the freezing point.
b. A volatile solute that does not dissociate:
In this case, the solute does not dissociate but remains as discrete molecules in the solvent. Since the solute is volatile, it has the tendency to escape into the gas phase more readily. As a result, the concentration of solute molecules in the solution remains relatively low. Consequently, the freezing point depression is not significant compared to when a nonvolatile solute is added.
c. Two solutes that react according to the equation: A+B --> C:
If two solutes react to form a new compound or product (C), the effective concentration of both solutes will change. The change in the concentrations of the reactants will determine the freezing point depression. For example, if both reactants have a lower concentration compared to the original solvent, the freezing point depression may be less significant. However, if the reaction generates additional particles or ions, increasing the overall concentration, it will contribute to a greater depression of the freezing point.
In summary, the freezing point depression depends on the nature of the solute, whether it dissociates or remains as discrete molecules, as well as the concentration of solute particles or the reactants involved in the solution.